1. Field of the Invention
The present invention relates to a liquid crystal display device, more particularly, to a liquid crystal display device using a ferroelectric liquid crystal.
2. Description of the Related Art
Conventional liquid crystal display devices mainly use nematic liquid crystals. The nematic liquid crystal display devices include Twisted Nematic (TN) type and Supertwisted Birefringence Effect (SBE) type. The TN-type liquid crystal display device has a disadvantage that satisfactory contrast cannot be obtained because the driving margin has been reduced in accordance with the development of the driving mode to be multiplex. Likewise, the SBE-type liquid crystal display device, or an improvement of the TN-type liquid crystal display which makes use of a larger twist angle, has a disadvantage of reduced contrast and slow response when applied to a large-scale display. To overcome the foregoing shortcomings, N. A. Clark and S. T. Lagerwall proposed a liquid crystal display device using a chiral smectic C liquid crystal, i.e., a ferroelectric liquid crystal in 1980, as disclosed in U.S. Pat. No. 4,367,924 and Japanese Unexamined Patent Publication 56-107216 (1981).
Unlike conventional nematic liquid crystal display devices utilizing electrical tilt effect developed by the dielectric anisotropy of liquid crystal molecules, the proposed liquid crystal display device utilizes rotatory power for aligning the polarity of spontaneous polarization of a ferroelectric liquid crystal with the polarity of electric field. This liquid crystal display device is mainly characterized by bistability, memory function, fast response, and wide viewing angle. When the ferroelectric liquid crystal is injected into a cell having a thin gap, the helical structure of the ferroelectric liquid crystal is unwound by the effect of the substrate boundary, and the bistability appears wherein there exist two stable regions in which liquid crystal molecules 11 are tilted by +.theta. degrees 17 and by -.theta. degrees 18 with respect to the normal of a smectic layer 12, as shown in FIG. 2(a). The application of electric field 16 to the ferroelectric liquid crystal in the cell allows the liquid crystal molecules 11 and their spontaneous polarization 15 to have a uniform orientation. Accordingly, the orientation of the liquid crystal molecules 11 can be switched from one state to the other by switching the polarity of the electric field to be applied.
The switching operation alters the birefringence of the ferroelectric liquid crystal in the cell and, hence, the light transmission can be controlled by interposing the cell between a pair of polarizers. In addition, even after the application of the electric field ceases, the liquid crystal molecules 11 retain the previous orientation, affected by the orientation restricting force of the substrate boundary, thereby exhibiting a memory effect. Furthermore, the direct interaction between the spontaneous polarization of the liquid crystal and the electric field provides a fast response display capability, wherein the time required for switching in the ferroelectric liquid crystal device is less than one thousandth of that required for switching in a nematic liquid crystal display device.
Researches are now being actively conducted concerning the applications of the ferroelectric liquid crystal having such excellent characteristic to a high-definition large-scale liquid crystal display device and a space modulation element for storing and processing light at a high speed.
However, the liquid crystal display device proposed by Clark and Lagerwall presents not few problems. Since the molecules of the ferroelectric smectic C phase liquid crystal have less symmetrical configuration and higher crystallization property than nematic liquid crystal molecules, it is difficult to uniformly orient the molecules in order to obtain a uniform display capability.
In an initial model of the ferroelectric liquid crystal display device, the smectic C phase has a layer structure of so-called book-shelf type wherein the smectic layers are perpendicular to the substrates as shown in FIG. 3, which illustrates a liquid crystal cell including substrates 9 and 10, smectic layers 13, and the normal line 12 of the smectic layers.
A cell fabricated utilizing a conventional orientation treatment such as rubbing presents unexpected switching phenomena and optical characteristics, that is, its switching characteristics are completely different from the proposed model. It has been identified by small-angle X-ray scattering analysis that this is partly attributed to a bending layer structure called chevron as shown in FIG. 4, wherein reference numeral 14 denotes a juncture of the chevron structure called chevron interface. See T. P. Rieker, N. A. Clark et al, Phys. Rev. Lett., Vol. 59, p 2658 (1987). Another point different from the initial model is that, though the orientation of spontaneous polarization is uniform, liquid crystal molecules assume a uniform orientation as well as a twisted orientation between upper and lower substrates as reported by Y. Ouchi, H. Takezoe and A. Fukada in Japan Journal of Application Physics, Vol. 26, p 1 (1987). In particular, the rubbing orientation treatment, in many cases, results in the twisted orientation of ferroelectric liquid crystal due to a strong restricting force on a boundary surface. In general, the twisted orientation does not create effective optical difference of the orientation of molecular axes at the switching between two different states, and cannot provide high contrast characteristic,
In order to realize the initial model proposed by Clark et al, some approaches have been proposed to eliminate such drawbacks. One of such approaches has been reported as attaining an oblique layer structure wherein the bending of the layers is prevented by providing a relatively large pretilt angle to the substrate prepared through SiO oblique evaporation technique. Another approach is to alter the layer structure into the book-shelf layer structure by applying a high-voltage alternating electric field to the cell of a bending structure, as proposed by Sato et at in the 12 th Liquid Crystal Symposium, Nagoya, 1F16 (1986). It has been reported that these two approaches obtained a high contrast characteristic.
However, the oblique evaporation of the first approach requires a difficult technique of forming uniform evaporation angle and a vacuum process, thereby presenting a critical production difficulty. In the second approach, it is also difficult to uniformly change the layer structure by the application of electric field, and the layer structure is, in most cases, gradually turned into the initial chevron structure over a long time. Therefore, this approach has not been put to a practical use yet.
To solve these problems, the present invention is to provide liquid crystal display element and device which present a high contrast characteristic despite the chevron structure thereof.